1. Field of the Invention
The present invention relates to distributed feedback semiconductor lasers and distributed Bragg reflector semiconductor lasers.
2. Description of the Related Art
Distributed feedback (referred to as “DFB” in the following) and distributed Bragg reflector (referred to as “DBR” in the following) are two well-known structures for semiconductor lasers, in which oscillation in a single longitudinal mode is achieved by employing a diffraction grating with excellent wavelength selectivity in the semiconductor optical waveguide. To use such semiconductor lasers as light sources for optical information processing or optical measurements, a so-called transversal mode control is necessary to ensure that the laser oscillates in the basic transversal mode.
For DFB semiconductor lasers, a structure for transversal mode control and current confinement is known, in which a stripe-shaped recess portion (so-called window) is formed in the current blocking layer, as disclosed for example in JP H06-112580A. The following is an explanation of this conventional structure, with reference to FIG. 11.
This DFB semiconductor laser includes an n-type GaAs substrate 101, and formed on top thereof an n-type GaAs buffer layer 102, an n-type Ga0.5Al0.5As first cladding layer 103, a Ga0.85Al0.15As active layer 104, a p-type Ga0.5Al0.5As first optical guiding layer 105, and a p-type Ga0.8Al0.2As diffraction grating layer 106. Formed on top of that are an n-type Ga0.4Al0.6As current blocking layer 109 having a stripe-shaped window 109a for current confinement, and a p-type GaAs protective layer 112. Furthermore, a p-type Ga0.5Al0.5As second cladding layer 110 and a p-type GaAs contact layer 111 are formed on top of the protective layer 112 and the stripe-shaped window 109a. 
In another well-known DFB semiconductor laser, a diffraction layer 106 having a diffraction grating 106a is formed below the active layer 104.
However, these conventional structures pose the following problems:    (1) The etching for the formation of the stripe-shaped window 109a degrades the shape of the diffraction grating 106a, and it is difficult to achieve single longitudinal mode characteristics with high yield.    (2) In the portion in which the thickness of the diffraction grating layer 109 has become thinner due to the formation of the diffraction grating 106a, over-etching may occur when forming the stripe-shaped window, and the etching may reach the first optical guiding layer 105 below the diffraction grating layer 106, reducing the yield.    (3) When the diffraction grating layer 106 is formed below the active layer 104, the diffraction grating is no longer exposed to the etching, but in that case the active layer 104 is formed on top of the diffraction grating layer 106 with its protrusions and recesses, so that it is difficult to achieve a stable crystal quality and variations in optical properties are increased.
Also known are DBR semiconductor lasers having a different structure to ensure single longitudinal mode oscillation. The following is an explanation of a method for manufacturing such a semiconductor, with reference to FIG. 12.
First of all, as shown in FIG. 12A, an n-type GaAs buffer layer 143, an n-type Ga0.5Al0.5As first cladding layer 144, a Ga0.85Al0.15As active layer 145, a p-type Ga0.5Al0.5As second cladding layer 146, and a grating layer 147 are formed on top of an n-type GaAs substrate 142 in a first crystal growth process. Then, as shown in FIG. 12B, a diffraction grating 147a is formed in the grating layer 147 in a distributed Bragg reflection region 141 by etching the grating layer 147 in this distributed Bragg reflection region 141.
Then, as shown in FIGS. 12C and 12D, a p-type Ga0.5Al0.5As third cladding layer 148 is formed in a second crystal growth process, and a ridge 148a extending in a direction perpendicular to the diffraction grating is formed by etching the p-type Ga0.5Al0.5As third cladding layer 148.
Then, as shown in FIG. 12E, a SiO2 film 150 is formed on the ridge 148a, and using this SiO2 film 150 as a mask for selective growth, an n-type GaAs current blocking layer 149 is formed on the p-type Ga0.5Al0.5As third cladding layer 148 in a third crystal growth process. Finally, as shown in FIG. 12F, the SiO2 film 150 is eliminated by selective etching, and a p-type GaAs contact layer 151 is formed in a fourth crystal growth process.
In the structure of FIG. 12F, a diffraction grating having a periodic structure is formed in the grating layer 147 in the distributed Bragg reflection region 141, so that single longitudinal mode oscillation is possible. Moreover, the forbidden band width of the n-type GaAs current blocking layer 149 is smaller than the energy of the laser light wavelength, so that the laser light is absorbed by the n-type GaAs current blocking layer 149 in the region outside the ridge 148a, the laser light is confined in the ridge 148a and laser oscillation in a single transverse mode is achieved.
However, in the conventional structure shown in FIG. 12, the grating layer is formed between the cladding layers, so that four crystal growth processes have to be carried out, which makes the manufacturing process complicated and lowers the yield. Furthermore, because of the optical absorption of the current blocking layer, the waveguide loss is high, and there is the problem that the operating current is large. In addition, because of the optical absorption of the current blocking layer, the optical distribution is limited, and there is the problem that it is difficult to achieve a large coupling coefficient.